Now that you have Fastlane correctly configured for both iOS and Android, created remote apps and added some first metadata it is a good time to actually do something with your Ionic or Cordova project: build it. For this you will leverage and edit the Fastfile for the first time and create new lanes in it. Exciting!
As with manual building, you have 3 options available how to build: 1) Via ionic, 2) via cordova or 3) native tooling (Android Studio, Xcode?). Use the linked article to choose which one best matches your requirements (TLDR: ionic for Ionic project, cordova for pure Cordova projects, native tooling if there is a specific reason) and then continue with the corresponding article:
When you have a Debug version of your app you can upload your app for testing (with HockeyApp or Testflight and Play Store Alpha). After testing and you succeeded in building a Release version of your app, publish it on the stores so your users can download it.